The present invention relates to the preparation of aromatic carbonates by carbonylation of an organic aromatic hydroxy compound, such as phenol, in the presence of a catalyst. More particularly, this invention relates to improved methods for the carbonylation of aromatic hydroxy compounds by a mixture of carbon monoxide and oxygen in the presence of a catalyst comprising palladium and a co-catalyst comprising lead.
Aromatic carbonates, such as diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents. This preparation requires economical access to large quantities of diaryl carbonates.
Various methods have been disclosed for the preparation of diaryl carbonates by the carbonylation of aromatic hydroxy compounds with carbon monoxide and oxygen in the presence of a catalyst system. The majority of the disclosures are directed to either batch-type processes, or to "batch flow" systems wherein all reactants other than gases are added to the reaction batch-wise, while gases are provided continuously throughout the reaction. As is well known, continuous flow processes are considerably more economical on a commercial scale than either batch-type or batch flow processes. There thus remains a need in the art for methods of carbonylation wherein diaryl carbonates may to produced in a continuous process in high yield.
In general, the carbonylation reaction requires a complex catalyst system, comprising at least a catalyst, a co-catalyst and a generally organic halide source. U.S. Pat. No. 4,187,242 to Chalk discloses catalysts selected from Group VIIIB metals, i.e., ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof. Further developments in the carbonylation of hydroxy aromatic compounds have focused primarily on the development of the co-catalyst and other elements of the catalyst system (the "co-catalyst package"). The yield and purity of the diaryl carbonate product vary widely depending on the identity of the co-catalyst and other elements. Thus the reaction conditions which optimize the purity and yield of the diaryl carbonates are different for each co-catalyst package.
Co-catalysts generally comprise a metallic species, for example cobalt, iron, cerium, manganese, copper, or lead, and an organic compound. Thus, for example, U.S. Pat. No. 5,142,086 to King, Jr., et al. discloses metallic co-catalyst selected from cobalt, iron, cerium, manganese, molybdenum, samarium, vanadium, chromium, and copper and an organic co-catalyst selected from aromatic ketones, aliphatic ketones and aromatic polycyclic hydrocarbons. U.S. Pat. No. 5,231,210 to Joyce et al. discloses a cobalt pentacoordinate complex co-catalyst and a quaternary onium salt. U.S. Pat. No. 5,284,964 to Pressman et al. discloses an inorganic co-catalyst selected from cobalt, manganese, and copper and organic co-catalyst selected from quaternary onium salts and terpyridine. Other co-catalyst systems include a divalent or trivalent manganese salt in combination with a tetraalkylammonium halide, as disclosed in EP 350,697 to Chang; a divalent or trivalent cobalt compound, tetraalkylammonium halide and a quinone as disclosed in EP 350,700 to Chang; or a copper compound, a quinone and onium halide as disclosed in U.S. Pat. No. 5,336,803 to Kezuka et al.
One catalyst system of particular interest is disclosed in U.S. Pat. No. 5,498,789 to Takagi et al. The catalyst system consists of a palladium compound, at least one lead compound, at least one halide selected from quaternary ammonium halides and quaternary phosphonium halides, and optionally at least one copper compound. Use of a lead co-catalyst yields a process wherein the yield of aromatic carbonate per palladium (turnover number of palladium) is high, i.e. greater than about 700. However, use of a lead co-catalyst also results in a low reaction rate, too low for commercial purposes. Accordingly, there also remains a need in the art for optimization and improvement of carbonylation of aromatic hydroxy compounds in the presence of catalyst systems comprising lead co-catalysts.